Recent cooling in Soviet-American relations has directed attention to the role of the president's special assistant for national security, Abigniew Brzezinski. He plays an advocate's role - something far different from any of his predecessors in the office.

He does not discipline the president or check his weaknesses, as the special assistant usually did. In the Carter administration, as a result, the role of safety man in national security has had to devolve haphazardly on somebody else.

The office of the special assistant for national security affairs developed in the postwar period as a mechanism for coordinating the divergent views of State, Defense, the Joint Chiefs, the CIA and other institutions with foreign-policy interests. Under Truman and Eisenhower, secretaries of State Dean Acheson and John Foster Dulles managed the big business, and the White House office largely shuffled papers.

McGeorge Bundy gave the office luster under Kennedy, but concentrated on refereeing rather than being a player. Although a passionate partisan on Vietnam, Walt Rostow emulated the Bundy model in other areas, mainly through the office of a fine deputy, Francis Bator.

Henry Kissinger brought the job front and center in the policy process, thanks chiefly to the weakness of William Rogers as secretary of state. But even Kissinger, before he moved to State, sought to lay out for the president in a scrupulously fair manner the positions of the various departments and agencies.

Like most of the Carter White House staff, Brzezinski is much more the enthusiastic partisan than the manager of a process. He is a passionate supporter of particular policy lines, and from his first days in office he has not been slow to make his views known to other officials, foreign diplomats, legislators and journalists. Unlike any of his predecessors, he has not taken on his staff the highly skilled analysts required for dissecting defense-budget matters or foreign economic policy.

The central theme of Brzezinski's advocacy derives from suspicion of, even aversion to, the Soviet Union. He was born a Pole, took consistently anti-Soviet positions during his graduate school days at Harvard, as a professor at Columbia and during a brief stint in the State Department during the 1960s.

Under Carter he has been tough on Russia in arms control, human rights and attitudes toward Western Europe, China and the Third World. Though he favored the "comprehensive settlement" approach to the Middle East - in part, I suspect, because it went against the grain of Kissinger's step-by-step approach - he did not lean to the joint U.S.-Soviet declaration, which that policy at one point entailed.

Brzezinski tends to play down his intimacy with Carter, but beyond doubt he serves the president in important ways. He is well known to Carter and trusted more than anybody else in the foreign-policy community. He ranges widely, has a gift for lucid explanation, and for the kind of historical analogy that makes presidents feel they are doing big things. For example, he likened Carter's stand on the Palestine homeland to the Balfour Declaration.

But his personality can be abrasive. He has alienated serveral foreign governments, including those of Israel and Russia. Several comments, including one made recently at the Great Wall of China - "last one to the top gets to fight the Russians in Ethiopia" - suggest a lack of nice judgment, and even good taste.

As an advocate, moreover, he shares Carter's cheif weakness: the inability to see how good intentions on one matter cen lead to bad results across the board. Thus a great many of the early blunders made by the Carter administration - notably the first arms-control proposal to Moscow - were mistakes made by Carter with the active help of the man who should have been saving him.

Presumably the president knows all this, and there is no convincing evidence that he is Brzezinski's man. But neither has Carter developed a system for saving himself from the mistakes to which he and Brzezinski are both prone. The safety man on foreign policy in the Carter akministration is whoever happens to come along.

Usually the task devolves on Secretary of State Cyrus Vance. He, for instance, saw the confusion building in Sino-Soviet relations and prompted the president's speech on the subject at the Annapolis xommencement. But though Vance usually wins on the showdowns, he is loath to challenge Brzezinski, an old friend. Even when he does win, he witness the Ammapolis speech, whichended up as a muddle requiring further explanation to determine where the president really stands.